1. Field of the Invention
The present invention relates to an electric actuator for transmitting drive power from an electric drive source through a drive power transmitting mechanism to a moving mechanism thereby to move the moving mechanism, and a method of controlling such an electric actuator.
2. Description of the Related Art
Heretofore, electric actuators have widely been used as a mechanism for transferring workpieces or the like.
As shown in FIG. 15 of the accompanying drawings, a conventional electric actuator 1 has a motor 2 as an electric drive source which is energized to cause a drive power transmitting mechanism 3 (e.g., a ball screw, a timing belt, or the like) to displace a slider 4 including a feed table or the like along a guide (e.g., a linear guide rail, a guide block, etc.) for thereby delivering a workpiece or the like placed on the slider 4 to a desired position.
In order to displace the slider 4, i.e., to control movement of the slider 4, a controller 5 outputs a drive signal to the motor 2 based on positional information of the slider 4 which is acquired by an encoder 6 and a command signal issued from a PLC (Programmable Logic Controller) 7 which controls an apparatus on which the electric actuator 1 is mounted. In FIG. 15, a power supply 8 supplies electric energy to the controller 5 and the motor 2.
When conditions for moving the slider 4 are to be set, i.e., when conditions including a stop position, a speed, and an acceleration are to be set, the user of the electric actuator 1 needs to enter numerical data of those conditions from an external input mechanism 9 such as a teaching box, a PC (Personal Computer), or the like into the controller 5. Specifically, as shown in FIG. 16 of the accompanying drawings, numerical data of a stop position is entered in step S101, and then numerical data of a speed and an acceleration are entered in step S102.
Thereafter, the PLC 7 and the controller 5 move the slider 4 in a test mode in step S103. Then, it is determined whether the movement of the slider 4 in the test mode is appropriate or not in step S104. Specifically, the user confirms the movement of the slider 4 in the test mode and determines whether the movement of the slider 4 in the test mode is appropriate or not. If the movement of the slider 4 in the test mode is judged as being appropriate (YES in step S104), then the slider is ready to be operated at an operation mode in step S105.
If the movement of the slider 4 in the test mode is judged as being inappropriate (NO in step S104), the process returns to step S102, and steps S102 and S103 are repeated. The setting of a stop position, a speed and an acceleration is repeated until the movement of the slider 4 in the test mode is judged as being appropriate (YES in step S104).
For setting a stop position of the slider 4 from the external input mechanism 9, the user usually measures the distance between stop positions of the slider 4 in advance, or calculates the distance between stop positions for the slider 4 based on positional information (the number of output pulses) from the encoder 6, and enters the measured or calculated distance as numerical data. For setting a speed or an acceleration of the slider 4 from the external input mechanism 9, the user enters desired numerical data of a speed or an acceleration, confirms the movement of the slider 4 in a test mode based on the entered numerical data, and if the movement of the slider 4 is not appropriate, changes the numerical data and reenters the changed numerical data. As described above, the process to set conditions for moving the slider 4 is highly complex because numerical data need to be manually entered and the movement of the slider 4 needs to be confirmed several times in a test mode.
The electric actuator 1 controls the slider 4 to stop at a stop position that has been set, based on positional information from the encoder 6. The accuracy with which to stop the slider 4 at the stop position is governed by the resolution of the encoder 6, i.e., the number of pulses output from the encoder 6 per unit distance. Therefore, if positional control with higher accuracy is required, it is necessary to employ a different encoder of higher resolution as the encoder 6, and also to employ a ball screw or the like with higher accuracy as the drive power transmitting mechanism 3. As a result, the cost of electric actuator 1 increases.
It is therefore a general object of the present invention to provide an electric actuator which allows moving conditions to be set easily and which has an inexpensive structure for positioning a moving mechanism at a stop position with increased positioning accuracy.
An electric actuator according to the present invention sets a stop position as a moving condition for a moving mechanism by positionally adjusting a positioning mechanism, and allows the user of the electric actuator to set at least one of a speed and an acceleration of the moving mechanism by operating a speed regulator or an acceleration regulator while confirming movement of the moving mechanism. Therefore, the user is not required to perform a complex process of entering numerical data of moving conditions, and can easily and reliably set moving conditions for the moving mechanism.
The electric actuator learns the distance that the moving mechanism has traveled, and moves the moving mechanism based on the learned distance and at least one of the speed during constant-speed movement of the moving mechanism and the acceleration during accelerated movement of the moving mechanism. For moving the moving mechanism to the start point or the end point, the moving mechanism is controlled to move at a speed lower than the speed in constant-speed movement and positioned at the start point or the end point. As a result, the moving mechanism can accurately be positioned.
Since the positioning accuracy of the moving mechanism is determined by abutment of the moving mechanism against the positioning mechanism, the moving mechanism can be positioned accurately by a positional information detector and a drive power transmitting mechanism which are relatively inexpensive without the need for a positional information detector and a drive power transmitting mechanism which are of high resolution and are highly expensive.
The positioning mechanism includes a shock absorber for damping shocks produced when the moving mechanism abuts against the positioning mechanism. Therefore, shocks produced when the moving mechanism abuts against the positioning mechanism can be reduced.
A method of controlling an electric actuator according to the present invention sets a stop position as a moving condition for a moving mechanism by positionally adjusting a positioning mechanism, and allows the user of the electric actuator to set at least one of a speed and an acceleration for the moving mechanism by operating a speed regulator or an acceleration regulator while confirming movement of the moving mechanism. Therefore, the user is not required to perform a complex process of entering numerical data of moving conditions, and can easily and reliably set moving conditions for the moving mechanism.
In the above method, the distance that the moving mechanism has traveled is learned, and the moving mechanism is moved based on the learned distance and at least one of the speed in constant-speed movement of the moving mechanism and the acceleration in accelerated movement of the moving mechanism. For moving the moving mechanism to the start point or the end point, the moving mechanism is controlled to move at a speed lower than the speed in constant-speed movement and positioned at the start point or the end point. As a result, the moving mechanism can accurately be positioned.
In the above method, the moving mechanism is stopped at an intermediate point between the start point and the end point according to a command from a control console based on the positional information from the positional information detector. The moving mechanism can be stopped not only at the start point and the end point, but also at the intermediate point between the start point and the end point. Thus, the moving mechanism can move or transfer a workpiece or the like with increased freedom.
In the above method, the control console monitors the distance that the moving mechanism travels which is calculated based on the positional information from the positional information detector and the speed of the moving mechanism, and limits a drive signal output to an electric drive source after the moving mechanism reaches the start point or the end point if the monitored distance that the moving mechanism travels is judged as reaching the distance learned by a learning device or if the monitored distance that the moving mechanism travels is judged as not reaching the distance learned by the learning device and the speed of the moving mechanism is equal to or lower than a predetermined speed. Thereafter, after the moving mechanism has reached the start point or the end point, the electric drive source is prevented from generating an excessive torque. As a result, the electric drive source, a power transmitting mechanism, and the start and end points are prevented from being subjected to an excessive load. The electric actuator thus has its durability increased.
In the above method, the electric drive source comprises a motor, and the control console monitors the distance that the moving mechanism travels which is calculated based on the positional information from the positional information detector, and resets a deviation between the drive signal and the monitored distance that the moving mechanism travels if the deviation exceeds a predetermined range after the moving mechanism reaches the start point or the end point. The positioning accuracy at the stop position of the moving mechanism is increased, and the electric drive source, the power transmitting mechanism, and the start and end points are prevented from being subjected to an excessive load. The electric actuator thus has its durability further increased.
In the above method, the electric drive source comprises a motor, and the control console rotates the motor alternately in a normal direction and a reverse direction under open-loop control to synchronize the positional information from the positional information detector and positional information corresponding to a rotational angle of the motor with each other when a power supply of the electric actuator is turned on. Consequently, the position to which the moving mechanism is moved can accurately be detected according to the positional information based on the rotational angle of the motor, maximizing the capability of the motor.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.